EE customers have been overcharged and left hundreds of thousands of pounds out of pocket after a billing mistake.

Communication regulators Ofcom fined the mobile giant a whopping £2.7 million for breaking what they called a “fundamental billing rule” on two separate occasions.

Nearly 40,000 EE customers were overcharged by £250,000.

Lindsey Fussell, Ofcom's consumer group director, said: "EE didn't take enough care to ensure that its customers were billed accurately. This ended up costing customers thousands of pounds, which is completely unacceptable.

"We monitor how phone companies bill their customers, and will not tolerate careless mistakes. Any company that breaks Ofcom's rules should expect similar consequences."

Initially the company decided not to reimburse most customers until Ofcom intervened.

While Ofcom recognised EE had not intended to profit from the overcharging they had shown “carelessness or negligence”.

EE customers who called the company's customer services number, 150, while roaming within the EU were incorrectly charged as if they had called the United States.

Ofcom said the mistake saw customers charged £1.20 per minute, instead of 19p per minute.

In another breach, despite making it free to call or text the 150 number from within the EU from November 18, 2015, EE kept billing 7,674 customers up until January 11, 2016.

EE, which are owned by BT following a £12.5 billion deal last year, accepted full responsibility.

Nearly 7,000 customers couldn't be identified for refunding according to EE, meaning they are still £60,000 worse off in total.

EE donated the excess fees it collected from customers it couldn't trace to charitable causes as per Ofcom's guidlines.

An EE spokesman said: "We accept these findings and apologise unreservedly to those customers affected by these technical billing issues between 2014 and 2015.

"Following Ofcom's findings, we have made a number of additional improvements to our systems and policies to allow us to better support our customers in the rare occasion that billing issues do occur."

Ofcom has told EE it must take further action to trace and refund every customer involved.

If you think you might have been affected you should check your latest statements and make a formal complaint to your service provider.